1. Field of the Invention
This invention relates to a rotary sensor and more particularly to a rotary sensor capable of detecting rotation angle with high reliability.
2. Description of Related Art
In a prior art rotary sensor, as shown in FIG. 8, a housing 1 is formed in an approximately cylindrical shape surrounding the outer periphery. In this housing 1 a partition wall 1a is formed.
On the right side surface of the partition wall 1a in the drawing a resistor board 2 is positioned by the use of an annular spacer 3 and fixedly attached to the partition wall 1a.
On one end of the resistor board 2 a terminal 4 is attached. The terminal 4 is projectively formed on the left side of the partition wall 1a in the drawing. A sealant 1b is filled in a gap portion of the partition wall 1a where the terminal 4 is attached, to thereby separate the partition wall 1a to the right and left members in the drawing.
In the vicinity of the forward end of the terminal 4 the end of a lead 4 is soldered. The lead is led out upwardly in the drawing.
On the surface of the resistor board 2 a resistor pattern not depicted is formed by printing. A slider piece 6 which slides in elastic contact with the resistor pattern is attached on a slider holder 7, which is rotatably disposed oppositely to the resistor board 2. Between the outer periphery of the slider holder 7 and the annular spacer 3 there is mounted a thrust washer 8 to reduce frictional resistance.
The slider holder 7 is rotatably mounted with the hollow portion of its boss portion 7a formed on the center of rotation inserted over a stationary shaft 9a of a holder guide 9. On the upper right side surface of the slider holder 7 in the drawing, a lock pawl 7b is formed projecting to the holder guide 9 side.
On the outer periphery side of the boss portion 7a one torsion coil spring 10 is mounted. The torsion coil spring 10 is attached at one end to the slider holder 7 side and at the other end to the holder guide 9 side, so that the slider holder 7 will constantly be pressed to rotate in one direction.
The holder guide 9 is securely attached at the outer peripheral side surface to the inner peripheral surface of the housing 1, and a circular long hole 9b as viewed from the front is formed above the stationary shaft 9a in the drawing, thus schematically constituting the prior art rotary sensor.
The prior art rotary sensor, when used as an accelerator pedal position sensor of an automobile for example, is attached on a predetermined mounting member not depicted. On the right side of the rotary sensor in the drawing, a rotatable drive shaft 11 is disposed and coupled with the automotive accelerator pedal not depicted. On the forward end of the drive shaft 11 an arm member 12 having an approximately L-shaped retaining portion 12a is fixedly mounted.
The forward end of the retaining portion 12a is positioned within the long hole 9b of the holder guide 9, and is constantly in elastic contact with the lock pawl 7b of the slider holder 7 which is pressed to rotate in one direction by the elastic force of the torsion coil spring 10.
When for instance the accelerator pedal is depressed to turn the drive shaft 11 through a specific angle in the other direction, the slider holder 7 is also turned through a specific angle in the other direction against the elastic force of the torsion coil spring 10.
With the rotation of the slider holder 7, the resistance value of an unillustrated resistor on the resistor board 2 varies. This variation in the resistance value is detected by an unillustrated control unit connected to the lead wire 5, thereby enabling detection of the rotation angle of the drive shaft 11.
In the meantime, when the accelerator pedal is released, the drive shaft 11 is restored to the initial state and, with the rotation of the arm member 12, the slider holder 7 is reset to the initial position by the elastic force of the torsion coil spring 10.
In the prior art rotary sensor described above, however, only a single torsion coil spring 10 is employed to elastically force the slider holder 7 towards the direction of rotation. The torsion coil spring 10, therefore, is subjected to accumulated metal fatigue on the slider holder 7 side or in the retaining portion on the holder guide 9 side, resulting in a spring failure during rotation of the slider holder 7 after a use for long-time. In the event of the failure of the torsion coil spring 10, the slider holder 7 can not be restored to the initial position and consequently the automobile will fail to operate.
Also it is necessary to provide a force of restoration of a specific value or more in any position of rotation. Where the torsion coil spring 10 alone is used, it becomes necessary to increase a spring constant, which, however, will make the torsion coil spring 10 liable to break. The problem also arises that the torsion coil spring 10 of the rotary sensor requires an increase in the strength of its mounting portion.
As a measure to solve these problems, the number of coils of the torsion coil spring 10 needs to be increased to lower the rate of increase of the spring force with respect to the amount of rotation. In this case however, the problem arises that the rotary sensor will increase in height.